1. The Field of the Invention
The present invention relates generally to optical amplifiers for amplifying a plurality of optical wavelength channels. More specifically, the invention relates to equalizing gain in an optical amplifier for a plurality of optical wavelength channels.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. Light signals allow for high transmission rates and high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber-optic cable as can occur with electronic signals in wire-based systems. Light signals also can be conducted over greater distances without the signal loss typically associated with electronic signals on wire-based systems.
While signal loss in a fiber-optic cable is less than that in wire-based systems, there is nonetheless some signal loss over the distances that light signals may be transmitted. To compensate for the signal loss, optical amplifiers are used. Two common optical amplifiers are Raman amplifiers and Erbium Doped Fiber Amplifiers (EDFAs). Both of these amplifiers use characteristics of doped fiber-optic cables to amplify light signals.
The amplifier pumps light onto the fiber-optic cable where the light is at a different frequency than the light signal that is to be amplified. As the light signal and pumped light travel along the fiber-optic cable, energy from the light that is pumped onto the fiber-optic cable is transferred to the light signal. Optical amplifiers use optical pumps, i.e. laser sources, to generate the light that is pumped into the fiber-optic cable.
In some fiber-optic applications, multiple signals may be sent simultaneously by using different wavelengths of light. Each wavelength may be referred to as a channel. For example, the C-band might be used to transmit 40 different channels or wavelengths along the 1530 to 1562 nm bandwidth. In a variety of optical applications it is desirable to amplify each channel with about the same optical gain. However, the optical gain of an optical gain medium, such as the doped fiber-optic cables, depends upon wavelength. Some wavelength channels will experience greater amplification than others. Consequently, a single gain medium does not usually function as a high gain medium having substantially uniform optical gain over an extended wavelength range.
Conventional approaches to providing uniform optical gain over an extended wavelength range typically have more components than desired, require significant numbers of optical interconnects resulting in insertion losses, and typically cost more than desired. Illustratively, EDFAs are widely used to amplify optical signals to compensate for transmission losses and insertion losses caused by interconnection of components and the gain characteristics of EDFAs are a strong function of optical wavelength. Therefore, to achieve substantially uniform optical gain over an extended wavelength for an EDFA requires an additional gain equalization filter (GEF). In a single stage amplifier, GEFs are commonly placed after the final stage of the amplifier. For multi-stage amplifiers, GEFs are sometimes placed between amplifier stages. Each GEF introduces an additional component cost, component size, and requires appropriate packaging to permit it to be optically coupled to other components. Further, physically coupling components together results in some degree of insertion loss for each physical connection.